1. Field of the Invention
The present invention relates to a method of deforming a resist pattern to be used for forming a semiconductor device, and more particularly to a method of improving the accuracy in the quantity of deformation of an original resist pattern or improving a highly accurate control to a pattern shape of a reflow-deformed resist pattern.
2. Description of the Related Art
A conventional well known method of deforming the original resist pattern is a re-flow process by heating the original resist pattern. A quantity of deformation of the resist pattern or a difference in size of the deformed resist pattern from the original resist pattern is relatively small, for example, in the range of 0.5 micrometers to 3 micrometers.
Another conventional well known method of deforming the original resist pattern is to dip the original resist pattern into chemicals or expose the original resist pattern to a steam containing chemicals so that the chemicals osmose into the original resist pattern, whereby the original resist pattern is dissolved and deformed. A quantity of deformation of the resist pattern or a difference in size of the deformed resist pattern from the original resist pattern is relatively large, for example, in the range of 5 micrometers to 20 micrometers.
A high accuracy in the quantity of deformation of the resist pattern is desired. In order to obtain the high accuracy in quantity of the deformation, a highly accurate control to the quantity of deformation of the resist pattern is essential.
A conventional method of forming a thin film transistor utilizes the original resist pattern and the deformed resist pattern. FIG. 1A is a fragmentary plan view of a thin film transistor of a first step involved in conventional sequential fabrication processes. FIG. 1B is a fragmentary cross sectional elevation view of a thin film transistor shown in FIG. 1A, taken along a D-D′ line. FIG. 2A is a fragmentary plan view of a thin film transistor of a second step involved in conventional sequential fabrication processes. FIG. 2B is a fragmentary cross sectional elevation view of a thin film transistor shown in FIG. 2A, taken along a D-D′ line. FIG. 3A is a fragmentary plan view of a thin film transistor of a third step involved in conventional sequential fabrication processes. FIG. 3B is a fragmentary cross sectional elevation view of a thin film transistor shown in FIG. 3A, taken along a D-D′ line. FIG. 4A is a fragmentary plan view of a thin film transistor of a fourth step involved in conventional sequential fabrication processes. FIG. 4B is a fragmentary cross sectional elevation view of a thin film transistor shown in FIG. 4A, taken along a D-D′ line. A thin film transistor is formed over an insulating substrate 301.
With reference to FIGS. 1A and 1B, a metal layer is formed on a top surface of an insulating substrate 301. The metal layer is then patterned to form a gate electrode 302. A gate insulating film 303 is formed over the top surface of the insulating substrate 301 and over the gate electrode 302. An amorphous silicon film 304 is formed over the gate insulating film 303. An n+-type amorphous silicon film 305 is formed over the amorphous silicon film 304. A metal layer 306 is formed over the n+-type amorphous silicon film 305.
Thick resist masks 318 and thin resist masks 328 are selectively formed over the metal layer 306. The thick resist masks 318 are adjacent to a channel region 315. The thick resist masks 318 separates the thin resist masks 328 from the channel region 315. The thick resist masks 318 have a thickness of about 3 micrometers. The thin resist masks 328 have a thickness of about 0.2-0.7 micrometers. Each pair of the thick resist mask 318 and the thin resist mask 328 comprises a unitary-formed resist mask which varies in thickness.
With reference to FIGS. 2A and 2B, a first anisotropic etching process is carried out by using the thick and thin resist masks 318 and 328 for selectively etching the metal layer 306 and the n+-type amorphous silicon film 305, whereby the remaining parts of the n+-type amorphous silicon film 305 become a source side ohmic contact layer 310 and a drain side ohmic contact layer 311, and further the remaining parts of the metal layer 306 become a source electrode 313 and a drain electrode 314.
A plasma ashing process is carried out in the presence of O2 plasma for reducing the thickness of the resist masks, whereby the thin resist masks 328 are removed, while the thick resist masks 318 remain with a reduced thickness. These thickness-reduced resist masks 318 will hereinafter be referred to as residual resist masks 338. The residual resist masks 338 are adjacent to the channel region 315. These residual resist masks 338 provide the original resist patterns.
With reference to FIGS. 3A and 3B, the residual resist masks 338 are exposed to a steam for 1-3 minutes, wherein the steam contains an organic solvent, whereby the organic solvent gradually osmose into the residual resist masks 338 as the original resist patterns, so that the original resist pattern is dissolved and re-flowed, resulting in a reflow-deformed resist pattern 348 being formed. The reflow-deformed resist pattern 348 extends to the channel region 315 and outside regions of the residual resist masks 338 as the original resist patterns.
In the re-flow process, the residual resist masks 338 as the original resist patterns are inwardly re-flowed toward the channel region 315 and the re-flowed residual resist masks 338 come together over the channel region 315. An interconnection 302′ connected to the gate electrode 302 extends in a parallel direction to the line D-D′. This interconnection 302′ forms a step-like barrier wall 317-a to stop the reflow of the re-flowed residual resist masks 338, wherein the step-like barrier wall 317-a extends in the parallel direction to the line D-D′. A further step-like barrier wall 317-b is present, which extends in a perpendicular direction to the D-D′ line.
For this reason, the reflow of the residual resist masks 338 is stopped but only in two directions by the step-like barrier walls 317-a and 371b. The reflow of the residual resist masks 338 is free and not limited in the remaining directions. It is difficult to control the reflow of the residual resist masks 338 in the remaining directions due to the absence of any re-flow restrictor such as the step-like barrier walls 317-a and 371b. This means it difficult to control the pattern shape of the reflow-deformed resist mask 348.
With reference to FIGS. 4A and 4B, a second anisotropic etching process is carried out by use of the reflow-deformed resist mask 348 and the source and drain electrodes 313 and 314 as masks for selectively etching the amorphous silicon film 304, whereby the remaining part of the amorphous silicon film 304 becomes an island layer 324. A pattern shape of the island layer 324 is defined by the reflow-deformed resist mask 348 in combination with additional masks of the source and drain electrodes 313 and 314. The used reflow-deformed resist mask 348 is removed. As a result, a reverse staggered thin film transistor is formed.
As described above, the pattern shape of the island layer 324 is defined by the reflow-deformed resist mask 348 in combination with additional masks of the source and drain electrodes 313 and 314. Further, it is difficult to control the reflow of the residual resist masks 338 in the remaining directions due to the absence of any re-flow restrictor such as the step-like barrier walls 317-a and 371b. It is difficult to control the pattern shape of the reflow-deformed resist mask 348. This means it difficult to control the pattern shape of the island layer 324. The island layer 324 of amorphous silicon underlies the source and drain sides ohmic contact layers 310 and 311. The island layer 324 is thus electrically connected to the source and drain electrodes 313 and 314. A parasitic capacitance between the gate electrode 302 and the source and drain electrodes 313 and 314 depends on the pattern shape of the island layer 324. In order to precisely control the parasitic capacitance, it is essential to control the pattern shape of the reflow-deformed resist mask 348 or to control the pattern shape of the island layer 324.
In the above circumstances, the development of a novel improving a highly accurate control to a pattern shape of a reflow-deformed resist pattern free from the above problems is desirable.